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Full text of "Text Book Of Mechanical Engineering"

List of Efficiencies.                           577

work transmitted = load on A x distance travelled on A
and load on A = half that on E,

1                             E               T)   XT                 A   IT    t,         E 7T R N

.-. work per mm. ==  x 27rRN        and H. P. =
2

_
F counterbalances the lever weight.

List   of  Efficiencies.  Efficiencies   in   various   cases, as

found by the methods previously described, are  as follows :

Cranes worked by spur gearing...        ...        ...        ...    30% to 6o/0

Worm gearing (indifferently constructed)       ...        ...    30%

Worm gearing (very carefully constructed)     ...        ...    9O/0

Weston pulley block, well greased      .........    30% to 4O/0

Screw jack          ..................    iS/o to 35%

Cornish engine(Brake H. P. -f Indie. H.P.) ......    35/0 to 6o/0

Other engines (Marine, Loco, Gas, Oil, &c.) ......    75/0 to ^5/0

Undershot water wheels ...............    25% to 30%

Overshot water wheels  ......       .........    77o to 75/o

Breast wheels (Poncelet floats)            .........    6o/0 to 65%

Pelton water wheel        ...............    8o/B to 9O*/0

Turbines (foil .sluice)      ...............    6o/0 to 8o/0

Hydraulic press (neglecting pump)      ...        ...        ...    9^/o t 99%

Hydraulic jack with pump       ......                  ...    77%

Pumps (piston)    ..         ......        .........    7&%

Hydraulic accumulator ...        ...        ...        ...              9 * %

Hydraulic lift, working rapidly           .........    50%

Hydraulic cranes, all losses taken       .........    55%

Mechanical efficiency of engines, not varying appreciably with
load, is often found by comparing an indicator diagram taken
' running light/ with that under working conditions. (See Ap^
pendix IV., p. 962.) (See pp. 874 and 1125.)

We will close this chapter with a few comparisons of power
transmitters:

COMPARISONS OF THE ADVANTAGES AND DISADVANTAGES OF   '
TRANSMUTING POWER BY VARIOUS METHODS.

Advantages.                                     Disadvantages.

i. LINK WORK.

Useful in modifying power and
obtaining special motions, as with
valve gear, parallel motions, &c.
Coupling rods a case of pure tians-

mission.

Frictional loss slight

Dead points often occur^ to be over-
come by force or chain closure.

Will only transmit over very short
spaces.

but depends on number of joints.